[llvm-dev] InductiveRangeCheckElimination and BranchProbabilityInfo

[Sam Parker via llvm-dev]

Hi,

extractRangeChecksFromBranch uses BranchProbabilityInfo to decide whether its worth trying the InductiveRangeCheckElimination transformation. For the following example:

void split() {
  for (int i = 0; i < 100; ++i) {
    if (i < 99)
      do_something()
    else
      do_something_else()
  }
}

But the reported BPI is reported as 50/50 to whether do_something will be called, but we can see with our human eyes that this should be 99%.

So two questions:
- why is BPI used to enable the transformation?
- would it not be more useful for BPI to use something like inductive range analyis to calculate the probability? And if so, what else could make use of it? To me, InductiveRangeCheckElimination feels like it should be separated out into an analysis and the transformation.
- or have I misunderstood how and what BPI does?

Thanks,
sam


Sam Parker

Compilation Tools Engineer | Arm

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Arm.com

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